1. Field of the Invention
The present invention relates generally to arc welding equipment and, more particularly, is concerned with an apparatus and method for accurately setting the distance, or arc gap, between a weld electrode and a workpiece in preparation for a welding operation.
2. Description of the Prior Art
Nuclear fuel and core control rods are fabricated from elongated hollow metal tubes being closed at their opposite ends by metal end plugs. These rods require high-integrity sealing of the end plugs to the tube ends to prevent in-service leakage, and to maintain internal gas pressure introduced at the time of fabrication. The end plugs are typically sealed to the ends of the tubes by using TIG (tungsten inert gas) fusion welds under helium or argon cover gas.
High quality TIG welding practice requires, among other things, close control of the physical distance, commonly called "arc gap", between the tip of the nonconsumable electrode and the workpiece. While conventional weld fixtures in many cases incorporate mechanisms for establishing proper arc gaps, for example linear slides, macroscopes with reticles and dial indicators, the actual setup and online monitoring and control of arc gap has heretofore usually been under manual control of the welding operator.
There are a number of problems with manual control. First, mechanisms to position and clamp the workpiece in place do not always achieve the desired level of accuracy and repeatability from piece-to-piece. Because it is time-consuming to do so, setting and fixing arc gap on each production workpiece is not feasible. Therefore, initial electrode gap setups remain in effect for a number of consecutive workpieces. Variability in workpiece positioning means that the resulting arc gaps are also variable.
Second, on highly automated lines, the line operator is not physically stationed at the welder at all times. As a consequence, the arc gap cannot be closely monitored, adjusted and otherwise controlled. Third, the weld electrodes erode slightly over time, thus increasing the originally preset arc gap.
Fourth, the weld electrodes used in pressurized seal welding accumulate a "ball" of weld metal on their tip. The tip ball grows and moves unpredictably from weld-to-weld, thus causing wide variations in effective arc gap. Fifth, in a number of applications, no gaging is in place for indicating preset arc gap. The gap is set according to the visual judgment of the welding operator, and is thus inaccurate and nonrepeatable.
Sixth, variability in arc gaps and use of "non-optimum" arc gaps reduce electrode life. Seventh, arc gaps which depart from a reasonably "safe" range can produce product weld quality problems, including arc-outs, shallow underpenetrated welds, tungsten inclusions from arc strikes in the molten weld puddle, etc. In addition to lowering product yields and increasing the cost of rework, undetected weld anomalies can lead to product service problems.
Consequently, a need exists for improvement of setting the arc gap in preparation for each welding operation in a way that substantially eliminates the necessity for operator skill and intervention. U.S. Pat. No. 4,439,664 to Toohey discloses an arc welding apparatus wherein physical contact of the weld electrode with the workpiece is mechanically sensed. In the Toohey apparatus, a lead screw which normally stationarily rotates to advance the weld electrode toward the workpiece is capable of a recoil movement that trips a microswitch upon the electrode contacting the workpiece. The tripped microswitch sends a signal to a control unit causing the electric motor driving the lead screw to reverse rotation for a predetermined period of time, thereby retracting the electrode a predetermined distance from the workpiece. The control unit can be set to apply voltage to initiate the welding arc automatically upon the electrode reaching the predetermined distance from the workpiece.
While the arc welding apparatus of the Toohey patent appears to be a step in the right direction, there is a continuing need to explore other design alternatives which may more effectively provide a solution to the above-described problems.